In order to pursue our overall objective of finding specific means of altering the expression of the transformed genome of mammary epithelial cells, whether they be preneoplastic or neoplastic, we have a) studied the influence of dietary lipids on the growth rate of transplantable mammary adenocarcinomas in both virus-infected and uninfected mouse strains as well as in rats, b) performed membrane studies with isolated cells from both normal and tumor murine mammary tissues, and c) developed cell surface markers which can distinguish epithelial cells, adipose cells, and fibrocytes from each other when grown in culture. Our results suggest that the tumor promoting effect of dietary fat a) is caused by cis, cis-linoleate; b) varies quantitatively with species and strain of animal; c) cannot be accounted for by an increased neoplastic cell proliferation and therefore, d) may be mediated through the cellular immune system.